In hybrid vehicles, regenerative braking is commonly used for charging a battery, namely, the battery is charged by the regenerative braking or by an engine. Fuel is consumed when the battery is charged by the engine while fuel is not consumed when the battery is charged by the regenerative braking. Therefore, power generation costs for charging the battery varies according to power sources. The power generation costs mean running costs for generating power. In the case that the power generation is performed by the engine, the power generation costs are determined by increases in fuel consumption for increasing engine power for the power generation or costs for the purchase of the fuel.
The fuel economy of the engine varies according to operating conditions of the vehicle in the case that the battery is charged with power generated by the engine. Namely, the power generation cost for charging the battery varies from time to time. Since the power generation costs vary depending on the power sources, the costs of power generation for supplying power to electrical loads in the vehicle vary at different times.
A method and apparatus for determining the cost of battery energy for a hybrid vehicle, which uses the battery energy at high rates, is proposed in U.S. Pat. No. 6,335,610B1 (JP-A-2002-118905). In this method, the cost of supplying charge energy to the battery is periodically computed. If the computation is performed in a long cycle, information regarding the previous battery charge greatly affects the computation. As a result, cost information is not properly updated in a timely manner after power generating conditions have changed. Moreover, a large size of memory is required for the computation.
If the computation is performed in a short cycle, the current cost information greatly affects the computation. As a result, the amount of energy previously charged in the battery is not reflected in the cost information. Furthermore, a usage of information on the computed battery energy cost is not proposed in U.S. Pat. No. 6,335,610B1.
The power generation cost for charging the battery can be reduced by economically managing the power generation. Effective power generation management however has not been provided in terms of fuel economy.